Yarn produced on rotor spinning frames differs in its bobbin building and unwinding behavior from ring-spun yarn. The rotor yarn is less napped than the ring-spun yarn and, therefore, is easier to unwind (greater ease of release). However, the rotor yarn has a greater curling tendency than the ring-spun yarn, causing the wound-up yarn in the edge areas of the cheese to be pushed outward by the yarn layers located on top. Because of this, a cheese can be formed which exceeds the normal bobbin traverse of, for example, 150 mm and can grow up to a width of 170 to 180 mm. Consequently, the desired bobbin buildup with level end faces is no longer possible. Such appearances occur with yarns made of natural fibers, such as cotton and, in particular, with coarse yarns. Coarser yarns produce a more marked distortion of the bobbin end faces.
Problems at the end faces of wound packages can already appear in the pre-stage of yarn production with wound-up fiber slubbing or roving yarn. U.S. Pat. No. 954,344 discloses that bulges will occur in the build-up of the end faces of wound packages if slubbings or rovings are not twisted or are only slightly twisted. This problem occurs if the winding angle is greater than 32°, which is customary in the prior art. In addition, the soft and loose structure of the fiber strands will contribute to the problem. Bulges can considerably impair the further processing of the wound packages. As disclosed in U.S. Pat. No. 954,344, the bulging can be prevented by increasing the winding angle in the end edge areas of the wound package, while keeping the winding angle the same in the rest of the areas of the wound package.
If cheese winding is performed at high yarn speeds, it is possible with medium and coarse yarns, because of the mass inertia of the yarn, for the yarn to move past the bobbin edge at the reversing points of the traverse and to create a so-called skipped yarn error. This fault leads to yarn breaks and hampers the further processing of the yarn.
The possibility of the occurrence of such errors is considerably affected by the crossing angle α. Therefore the selection of the appropriate crossing angle is of great importance when producing cheeses. When a cheese is produced with “random cross winding,” the yarn crossing angle remains constant over the entire bobbin travel. On the other hand, when a cheese is produced using “precision winding,” the yarn crossing angle is reduced as the cheese diameter increases. One advantage of precision winding over random cross winding is that precision winding produces a cheese with more running length, given the same bobbin volume. However, the crossing angle, which is reduced with increasing cheese diameter, limits the permissible maximum diameter when producing precision bobbins made of staple fiber yarns, since it is not possible to perform winding at arbitrarily narrow crossing angles with staple fiber yarns in order to avoid the defects occurring at the edges. For this reason, and as described, for example, in generic German Patent Publication DE 100 15 933 A1, crossing angles of less than 28° should be avoided in rotor spinning. Therefore, precision winding can only be used to a limited extent, particularly when winding staple fiber yarns.
In a third type of winding referred to as “step precision winding,” the goal is a crossing angle that remains approximately the same. In actual use, the above described density problems, or problems with the stability of the bobbin end edges, are somewhat reduced by means of step precision winding, but are not solved.
With conical cheeses which are driven by circumferential friction by a roller, it is necessary to let the drive be effective only within a predetermined area of a narrow friction zone, or of the friction zone of the cheese. Since the length of the bobbin circumference viewed along the bobbin axis differs, the number of revolutions of the cheese begins to fluctuate and becomes uncontrollable if the conical cheese to be wound comes into contact with the portions of the roller-shaped drive mechanism located to the left and right of the predetermined friction zone as the bobbin diameter grows. To prevent this occurrence, the yarn crossing angle in an area limited to the friction zone of a conical cheese is reduced in comparison with the yarn crossing angle outside of the friction zone as represented in German Patent Publication DE-AS 26 32 014, or in the parallel U.S. Pat. No. 4,266,734. By means of this reduction in the crossing angle, the compressive strength of the wound package is slightly increased in the winding zone. However, the only reason for forming a drive zone with increased specific pressure of the bobbin resting on the drive roller by changing the yarn crossing angle is to compensate for the different circumferential bobbin length in the case of conical cheeses.
New machine technology, in particular in weaving, such as air nozzle power looms, for example, make greater demands on the unwinding properties of the yarn. These requirements cannot be met, or are only insufficiently met, by means of the known bobbin embodiments.